The present invention relates to a method and a system for transmitting frequency-modulated radar waves to targets to calculate the distances to the targets.
As conventional systems of transmitting frequency-modulated radar waves to a target and receiving reflection waves therefrom to calculate the distance to the target, FMCW (Frequency Modulated Continuous Wave) radar systems have been known. The FMCW radar system is referred to simply as “FMCW radar” hereinafter.
One of the conventional FMCW radars has been disclosed in Japanese Patent Publication NO. H11-271432.
In the Patent Publication, the FMCW radar is configured to transmit a radar wave signal whose frequency, as shown in FIG. 11, is modulated so that the frequency is linearly repeatedly fluctuated like a triangular waveform with respect to time. The FMCW radar is configured to receive the radar wave signal that is reflected from the target and to mix the transmitted radar wave signal Ss1 with the received signal Sr1, thereby obtaining a beat signal having a frequency component corresponding to a difference between the transmitting frequency of the radar wave signal Ss1 and the received signal Sr1.
The FMCW radar is configured to obtain information related to the target according to the obtained beat signal.
Concretely, the FMCW radar executes the Fast Fourier Transformation (FFT) on the frequency components of the beat signal that correspond to a rising modulation period (sweep time ST) in which the frequency of the radar signal increases (rises), and on the remained frequency components thereof that correspond to a falling modulation period (sweep time ST) in which the frequency of the radar signal decreases (falls), thereby obtaining a power spectrum of the beat signal in each of the rising and falling modulation periods.
The FMCW radar samples a peak frequency component in each of the power spectrums and combines the sampled peak frequency components to obtain a pair of peak frequency components.
As shown in FIG. 11, assuming that one of the paired peak frequency components corresponding to the rising modulation period has a frequency of fb1, the other thereof corresponding to the falling modulation period has that of fb2, and the target moves at a relative velocity V over zero with respect to the FMCW radar, the FMCW radar applies the frequencies fb1 and fb2 to the following equations (1) to (4), thereby obtaining a distance (range) R1 from the FMCW radar to the target and/or a relative velocity V1 of the target:                     fr1        =                              fb1            +            fb2                    2                                    (        1        )                                fd1        =                              fb1            -            fb2                    2                                    (        2        )                                R1        =                              c            ·            fr1                                              4              ·              fm1              ·              Δ                        ⁢                                                   ⁢            F1                                              (        3        )                                V1        =                              c            ·            fd1                                2            ·            F01                                              (        4        )            where the fr1 represents a delay frequency from which the radar wave signal Ss1 is transmitted to which the reflected signal Sr1 is received, the ΔF1 represents a modulation width of the radar wave signal, the F01 represents a center frequency of the radar wave signal, the fd1 represents a Doppler frequency at which the frequency fb2 is shifted with respect to the frequency fb1, the c represents velocity of light, and the 1/fm represents each of the rising and falling modulation periods. Incidentally, the Tr1 in FIG. 6A represents a delay time from which the radar wave signal Ss is transmitted to which the reflected signal Sr1 is received.
In cases where a plurality of targets exist around the FMCW radar, the FMCW radar samples a plurality of first peak frequency components in the plurality of power spectrums corresponding to the rising modulation periods of the plurality of targets, and a plurality of second peak frequency components in the plurality of power spectrums corresponding to the falling modulation periods thereof.
Then, the FMCW radar combines every first peak frequency component with every second peak frequency component to obtain every pair of the first peak frequency components and second peak frequency components. The FMCW radar computes every distance R1 and/or every relative velocity V1 on the basis of the every pair of the first and second peak frequency components.
The FMCW radar repeatedly executes the above processes of obtaining every distance R1 and/or every relative velocity V1 on the basis of the every pair of the first and second peak frequency components.
Then, the FMCW radar decides that each of the extracted distances R1 and relative velocities V1 is obtained by correctly combining each of the first peak frequency components of each of the targets with each of the second peak frequency components of each of the same.
The above combining processes and the extracting processes, however, must require the enormous amount of computing.
In addition, the FMCW radar must obtain each of the beat signals in each of the rising modulation period and the falling modulation period so that the enormous amount of time must be required for collecting the beat signals.
The above problems may make difficult the installation of the FMCW radar in a vehicle.
That is, when installing the FMCW radar in a vehicle, it is necessary to detect each target around the vehicle in the shortest possible time so as to predict a probability that the vehicle will collide with each target, to avoid the collision of the vehicle and each target and so on.
However, because the FMCW radar requires the enormous amount of computing and the enormous amount of time to detect each target, it may be hard to detect each target within the shortest possible time required for predicting the probability that the vehicle will collide with each target so as to avoid the collision of the vehicle and each target and so on.